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Chapter 20
Magnetic Fields and Forces
Bar Magnet
 A bar magnet is a
permanent magnet in the
shape of a bar
 The symbol for the
magnetic field is B
 The magnetic field lines
can be deduced from the
pattern of the iron filings
 The filings are small,
needle-shaped, permanent
magnets
Section 20.1
Magnetic Field Lines
 The magnetic poles are
indicated at the ends of the
bar magnet
 Called north and south
 The magnetic poles are
analogous to positive and
negative charges
 The north poles of the
filings are attracted to the
south pole of the bar
magnet
Section 20.1
Magnetic Field from Current
 Moving charges
produce magnetic fields
 An electric current
consists of moving
charges, so it will
produce a magnetic
field
 The iron filings show the
magnetic field pattern
due to the current
Section 20.1
Right-Hand Rule
 Point the thumb of your
right hand in the
direction of the current
 You thumb will be
parallel to the wire
 Curling the fingers of
your right hand around
the wire gives the
direction of the
magnetic field
Section 20.1
Plotting Field Lines
 Field lines are three-
dimensional
 A large dot (•) indicates
the tip of the vector
when it points out of the
plane
 A cross (×) denotes the
tails of the vector when
it points into the plane
Section 20.1
Charges and Magnetic Fields
 The electric current can be modeled as a collection
of positive electric charges
 The charges would be moving with a velocity parallel
to the current direction
 The direction of the magnetic field is given by the
right-hand rule
 A positive charge moving to the left produces the
same magnetic field as a negative charge moving to
the right
Section 20.1
Magnetic Field and Current Loop
 Treat the loop as many
small pieces of wire
 Apply the right-hand
rule to find the field from
each piece of wire
 Applying superposition
gives the overall pattern
shown in fig. 20.9B
Section 20.1
Magnetic Forces & Bar Magnets
 To determine the total
force on the bar magnet,
you must look at the
forces on each pole
 The total force is zero
 The total torque is nonzero
 The torque acts to align
the bar magnet along the
magnetic field
Section 20.2
Magnetic Moment
 The bar magnet possesses a magnetic moment
 The bar magnet is similar to an electric dipole
 The poles of the magnet can be thought of as a sort of
“magnetic charge”
 The north pole of one magnet will attract the south
pole of another magnet
 Unlike poles attract
 Like poles will repel
 Similar to electric charges
Section 20.2
Comparing Electric and Magnetic
Fields and Forces
 There are many similarities in the behavior of electric
charges and magnetic poles
 Unlike charges and unlike poles attract
 Like charges and like poles repel
 North and south magnetic poles always occur in
pairs
 It is not possible to obtain an isolated magnetic pole
Section 20.2
Force on Moving Charge
 Magnetic force acts on individual charges
 The force depends on the velocity of the charge
 If the charge is not moving, there is no magnetic force
 If a positive charge, q, is moving with a given velocity in
an external magnetic field, then the magnitude of the force
on the charge is
 FB = q v B sin θ
 The angle θ is the angle between the velocity and the
field
Section 20.3
Right Hand Rule 2
 To determine the
direction of the force,
use right hand rule 2
 Point the figures of your
right hand in the
direction of the velocity
and curl them in the
direction of the field
 Your thumb points in the
direction of the force
Section 20.3
Motion of a Charged Particle
 Assume a charged
particle moves parallel
to the magnetic field
 The angle between the
velocity and the field is
zero
 Therefore, the force is
also zero
 Since sin θ = 0
 FB = q v B sin θ
Section 20.3
Motion of a Charged Particle, 2
 FB = q v B sin θ
 Assume a charged particle
moves perpendicular to
the magnetic field
 The angle between the
velocity and the field is
90o
 Therefore, the force is qvB
 The particle will move in a
circle
Section 20.3
Motion, 2, cont.
 The circle lies in the plane perpendicular to the
magnetic field lines
 The radius of the circle can be calculated from noting
there must be a centripetal force acting on the
particle
FB  FC

mv 2
qvB 
r
mv
r 
qv
Section 20.3
Motion of a Charged Particle, 3
 Assume a charged particle
moves neither parallel nor
perpendicular to the
magnetic field
 The angle between the
velocity and the field varies
 The path of the particle is
helical
 The charged particle will
spiral around the magnetic
field lines
Section 20.3
Right Hand Rules, Summary
 Right-hand rule number 1: Finding the direction of the magnetic
field from an electric current
 Place the thumb of your right hand along the direction of the current
 Curl your fingers; they will then give the direction of the magnetic field
as the field lines encircle the current
 Right-hand rule number 2: Finding the direction of the magnetic
force on a moving charge, q
 Point the fingers of your right hand along the direction of the velocity
 Curl your fingers in the direction of the field

Curl your fingers through the smallest angle that connects the velocity and
the field
 If q is positive, the magnetic force is parallel to your thumb. If q is
negative, the magnetic force is in the opposite direction
Section 20.3
Magnetic Force on a Current
 An electric current is a
collection of moving
charges, a force acts on a
current
 From the equation of the
force on a moving charge,
the force on a currentcarrying wire is
 Fon wire = I L B sin θ
 The direction of the force is
given by the right-hand rule
2
Section 20.4
Torque on a Current Loop
 A magnetic field can produce a torque on a current loop
 Assume a square loop with sides of length L carrying a
current I in a constant magnetic field
 The directions of the forces can be found from right-hand
rule 2
Section 20.5
Torque, cont.
 On two sides, the current is parallel or antiparallel to the
field, so the force is zero on those sides
 The forces on sides 1 and 3 are in opposite directions and
produce a torque on the loop
 When the angle between the loop and the field is θ, the
torque is
τ = I L2 B sin θ
 For different shapes, this becomes
τ = I A B sin θ
Section 20.5
Magnetic Moment
 For a current loop, the
magnetic moment is I A
 The direction of the
magnetic moment is either
along the axis of the bar
magnet or perpendicular to
the current loop
 The strength of the torque
depends on the magnitude
of the magnetic moment
Section 20.5
Mass Spectrometer
 Allows for the
separation of ions
according to their mass
or charge
 The ions enter with
some speed v
 They pass into a region
where the magnetic
field is perpendicular to
the velocity
Section 20.6
Mass Spectrometer, cont.
 The ions travel in a circle in the mass spectrometer
 The radius of the circle is mv/qB
 Ions with different masses will travel in arcs with
different radii
 Mass spectrometer can also be used to find the
composition of a material
 Measure the values of v, B and r
 Calculate q/m

Charge to mass ratio
Section 20.6
Hall Effect
 An electric current is
produced by moving
electric charges
 A particular value of
current can be produced
by positive charges
moving to the right or
negative charges to the left
 The Hall Effect can
distinguish between the
two options
Section 20.6
Hall Effect, cont.
 Place a current-carrying wire in a magnetic field directed perpendicular to the
current
 With positive charge carriers, an excess positive charge accumulates on the top
edge
 With negative charge carriers, an excess negative charge accumulates on the top
edge
 Measuring the potential difference distinguishes between positive or negative
charge carriers producing the current
Section 20.6
Ampère’s Law
 Ampère’s Law is useful
when the magnetic field
lines have a simple
symmetry
 Similar in approach to
Gauss’ Law for electric
fields
 Relates the magnetic
field along a path to the
electric current
enclosed by the path
Section 20.7
Ampère’s Law, cont.
 For the path shown in fig. 20.29, Ampère’s Law
states that
B
L  μo Ienclosed
closed
path
 μo is the permeability of free space
 μo = 4 π x 10-7 T . m / A
 If B varies along the path, Ampère’s Law cannot be
used
Section 20.7
Magnetic Field of a Long Straight
Wire
 Ampère’s Law can be used to find the magnetic field near
a long, straight wire
 B|| is the same all along the path
 If the circular path has a radius r, then the total path length
is 2 π r
 Applying Ampère’s Law gives
μo I
B
2π r
Section 20.7
Field from a Current Loop
 It is not possible to find a
path along which the
magnetic field is constant
 So Ampère’s Law cannot be
easily applied
 From other techniques, the
field at the center of the
loop is
μo I
B
2R
Section 20.7
Field Inside a Solenoid
 By stacking many loops
close together, the field
along the axis is much
larger than for a single
loop
 A helical winding of wire
is called a solenoid
 More practical than
stacking single loops
Section 20.7
Solenoid, cont.
 For a very long solenoid, it
is a good approximation to
assume the field is
constant inside the
solenoid and zero outside
 Use the path shown in the
figure
 Only side 1 contributes to
the magnetic field
Section 20.7
Solenoid, final
 The magnetic field inside the solenoid is given by
μo N I
Bsolenoid 
L
 For a solenoid with a length much greater than the
diameter
Section 20.7
Magnetic Materials
 Magnetic poles always come in pairs
 It is not possible to separate the poles of a magnet
 To understand why, the atomic origin of permanent
magnetism must be considered
Section 20.8
Motion of Electrons
 The motion of an electron
around a nucleus can be
pictured as a tiny current
loop
 The radius is approximately
the radius of the atom
 The direction of the
resulting magnetic field is
determined by the
orientation of the current
loop

Using right-hand rule 1
Section 20.8
Electron Spin
 The electron also produces a
magnetic field due to an
effect called electron spin
 The spinning charge acts as a
circulating electric current
 The electron has a spin
magnetic moment
 When an electron is placed
in a magnetic field, it will
tend to align its spin
magnetic moment with the
magnetic field
Section 20.8
Magnetic Field in an Atom
 The correct explanation of electron spin requires
quantum mechanics
 Confirms an atom can produce a magnetic field in
two ways
 Through the electron’s orbital current loop
 Through the electron’s spin
 The total magnetic field produced by a single atom is
the sum of these two fields
Section 20.8
Magnetic Field from Atoms, cont.
 Each atom produces a
current loop
 The collection of small
current loops acts as one
large loop
 This produces the magnetic
field in the magnetic material
 The current in each atomic
loop is very small, but the
large number of atoms
results in a large effective
current
Section 20.8
Atomic Magnets to Permanent
Magnets
 Not all atoms will actually be magnetic since the
current loops of different electrons can point in
different directions
 Their magnetic fields could cancel
 The total magnetic field will depend on how the
atomic magnetic fields are aligned
 A permanent magnet has the atomic fields aligned
Section 20.8
Isolated Magnetic Poles
 A bar magnet is
produced by a collection
of aligned atomic-scale
current loops
 Cutting the magnet in
half produces two new
complete bar magnets
 It is not possible to
produce an isolated
magnetic pole
Section 20.8
Magnetic Domains
 It is possible for the atomic
magnets in different regions
within a magnetic material to
point in different directions
 Called magnetic domains
 The arrangement shown is
equivalent to two bar
magnets
 Because the atomic
magnets are aligned in
opposite directions, this
would appear to be
nonmagnetic
Section 20.8
Properties of Magnetic Domains
 A material has two
domains of approximately
the same size
 Apply a magnetic field
from a bar magnet
 The domain aligned with
the magnetic field grows
at the expense of the other
domain
 The material now acts like
a bar magnet
Section 20.8
Refrigerator Magnet
Section 20.8
Earth’s Magnetic Field
 The Earth acts like a very
large magnet
 A compass needle aligns
with its north magnetic
pole pointing
approximately toward the
Earth’s geographic north
pole
 So the Earth’s geographic
north pole is actually a
south magnetic pole
Section 20.9
Earth’s Magnetic Field, cont.
 The location of the Earth’s south magnetic pole does
not correspond exactly with the geographic north
pole
 The Earth’s south magnetic pole moves slowly
 Currently at about 40 km/year
 The Earth’s magnetic field has completely reversed
direction
 The field is probably produced by electric currents in
the core
Section 20.9
Cosmic Rays
 Charged particles from space




are called cosmic rays
Their motion is affected by
the Earth’s magnetic field
At the equator, the particles
are deflected away from the
Earth’s surface
At the poles, the particles
follow a helical path and
spiral into the poles
They interact with the
Earth’s atmosphere and
produce aurora
Section 20.9
YouTube!
 Coronal Mass Ejection and Earth
Applications of Magnetism
 Magnetism is used by doctors, engineers,
archeologists, and others
 Applications include
 Blood-flow meters
 Relays
 Speakers
 Electric motors
 Bacteria
 Magnetic dating
Section 20.10
Blood-Flow Meter
 Measures the blood
velocity in arteries
 Blood contains ions
 A magnetic field is
applied to the artery
 The resulting potential
difference across the
artery can be measured
 Blood velocity can be
measured
Section 20.10
Relays
 The field produced by a
solenoid can be made
larger by filling it with a
magnetic material
 A magnetic force is
exerted on the moveable
part of the switch
 A small current through
the solenoid can control a
much larger current
through the switch
Section 20.10
Speakers
 A speaker uses a solenoid
coil wrapped around a
magnetic material
 The current varies
according to the amplitude
and frequency of the
music
 The solenoid vibrates,
causing the speaker cone
to produce sound
Section 20.10
Electric Motor
 A magnetic field can produce a torque on a current loop
 If the loop is attached to a rotating shaft, an electric motor is
formed
 In a practical motor, a solenoid is used instead of a single loop
 Additional set-up is needed to keep the shaft rotating
Section 20.10
Electric Generator
 Electric generators are closely related to motors
 A generator produces an electric current by rotating
a coil between the poles of the magnet
 A motor in reverse
Section 20.10
Magnetic Bacteria
 Magnetotactic bacteria
possess small grains of
iron called magnetosomes
 Each grain acts as a bar
magnet
 Bacteria use the
magnetosomes to orient
themselves with the
Earth’s magnetic field
 Allows them to determine
up and down
Section 20.10
Magnetic Dating
 Reversals in the
Earth’s magnetic
field can be
used to date
past events
 Dates of more
than 25
reversals are
known
Section 20.10
Velocity-Dependent Force
 The magnetic force exerted
on a moving charged particle
is dependent on its velocity
 Differs from gravitational
and electrical forces
 The two observers shown
both agree the particle is
accelerated, but only
observer 1 says there is a
magnetic force acting on the
particle
Section 20.11
Velocity-Dependent Force, cont.
 Special relativity solves the dilemma
 Observer 2 will actually say the particle experiences
an electric force
 This shows a deep connection between electric and
magnetic forces
 The connect is a critical part of the theory of
electromagnetism
Section 20.11